The present invention relates to television signal distribution, and more particularly to a method and apparatus for converting between different television signal formats while retaining a common control data stream.
Television signals are conventionally transmitted in analog form according to various standards adopted by particular countries. For example, the United States has adopted the standards of the National Television System Committee ("NTSC") while most European countries have adopted either PAL (phase alternating line) or SECAM standards. A family of multiplexed analog component ("MAC") systems, in particular C-MAC, D2-MAC, and D-MAC have been proposed, mainly for direct broadcast satellite service ("DBS") applications wherein television signals are broadcast directly from a satellite to an earth station operated by the end user. The various MAC systems differ mainly in how the digitized sound is realized, and all need more bandwidth than an NTSC signal. Entirely new color television systems, not constrained by the limitations of the systems noted above, may be developed for high definition television ("HDTV"). The main attributes of HDTV are a new aspect ratio of picture width to height, a substantially increased number of scan lines for more vertical resolution, and a greater video bandwidth for more horizontal resolution. Some proposed HDTV systems achieve a bandwidth comparable to that for NTSC television signals by using digital compression techniques.
The various television systems identified above are all incompatible. Therefore, a programmer such as Home Box Office ("HBO") that desires to serve markets in which various standards are used, must live with the added complexity of generating a plurality of incompatible signal formats for communication via a broadcast media such as satellite, cable, or terrestrial transmission. Since programmers of premium television services must control access to their signals in order to derive subscriber revenues, an access control data stream is transmitted with the television information to permit authorized subscribers to receive particular programs via a descrambler. In the past, the different television broadcast systems each used a different access control data stream to enable authorized subscribers to receive particular television programs. Thus, signal programmers must contend with different access control schemes as well as different broadcast formats. This requirement increases the complexity and cost of the equipment required by a programmer.
Access control systems for television signals provided in a single format are well known. One such system is disclosed in U.S. Pat. No. 4,613,901 to Gilhousen, et al entitled "Signal Encryption and Distribution System for Controlling Scrambling and Selective Remote Descrambling of Television Signals", incorporated herein by reference. In the Gilhousen, et al system, a unique encryption key stream is generated for scrambling a television signal. The scrambled television signal is transmitted together with control data including various cryptographic keys. At each descrambler authorized to receive a particular television signal, the encryption key stream is reproduced for use in descrambling the television signal.
Other selective subscription descrambling systems are disclosed in Gilhousen, et al U.S. Pat. Nos. 4,712,238 and 4,792,973. These patents provide improved systems for enabling descrambling of a received scrambled signal on an impulse-purchase basis. U.S. Pat. No. 4,634,808 to Moerder discloses a system for reproducing a key signal in a descrambler that is unique to the descrambler, and was used in encrypting a key signal that must be decrypted for use in descrambling a television signal.
It would be advantageous to provide a system for converting television signals of one format to one or more other formats while maintaining a common control data stream among the different formats. This would enable simplification of a television signal distribution system, by permitting the same hardware and software to be used to provide access control for television signals broadcast in any format.
The present invention provides a system having the aforementioned advantages.